As champagne corks popped for several Americans freed in a dramatic prisoner exchange with Iran last month, another U.S. citizen didn't get to enjoy the bubbly. He stayed behind in the same notorious Tehran prison from which the others were released.

Nearly a month after a prisoner deal between Washington and Tehran, the friends and family of business consultant Siamak Namazi — who holds degrees from Tufts and Rutgers universities and has ties to many Washington foreign policy insiders — fear he has been forgotten in the warm afterglow of last month's swap and are pressing the Obama administration to step up efforts to free him.

Story Continued Below

"Siamak was left behind," said Bijan Khajepour, Namazi's former business partner and a cousin by marriage. "He was as much innocent as all the other U.S. citizens who were in Iranian prisons. And we should not forget him."

Iranian intelligence agents arrested Namazi, 44, in mid-October. He had traveled to Iran in July to attend a funeral but was blocked from leaving the country. He is being held in Tehran's Evin prison, where he is subjected to constant interrogations and almost never allowed visitors.

Sources familiar with the case said that Secretary of State John Kerry has been pressing his Iranian counterpart, Foreign Minister Javad Zarif, for Namazi’s release. On Friday, five Iranian-American groups sent a letter to Kerry, obtained by POLITICO, urging him to "redouble your efforts" to win Namazi's freedom.

"On Namazi, this is obviously a case that we've been pressing for, we're continuing to press with the Iranians, and we will not give up until he can return home, as the others have, as well," said a senior administration official. "So this case is not resolved yet, but we won't give up."

The case illustrates the fraught nature of U.S.-Iranian relations, even after the January 16 implementation of the Iran nuclear deal. Iran released five American prisoners the same day, including Washington Post reporter Jason Rezaian, in return for the pardon or release of seven Iranian nationals charged or held in the U.S.

Namazi's friends worry that he is the latest pawn in a post-nuclear deal power game between the U.S. and Iran. Hardliners in Tehran are determined to prevent any further thaw in relations between the two countries. The Obama administration, meanwhile, may be unwilling to make new concessions for Namazi's release. Obama described last month's exchange of prisoners as "a one-time gesture to Iran given the unique opportunity offered by this moment and the larger circumstances at play."

Another concern is that, with the nuclear deal underway, the intense pressure Obama faced over the high-profile cases of Rezaian and others — which led to rare flash of presidential temper in July — has passed.

"What we don't want is for Siamak and his case to get forgotten in the euphoria of the others having been released," said one friend of Namazi.

Namazi's case was never part of the deal secretly negotiated by Kerry and another top State Department official, Brett McGurk, which led to last month's group release. Those talks began in late 2014, almost a year before Namazi's arrest last October.

At first, people close to Namazi initially hoped he might be released quickly without getting caught up in international diplomacy, sources said, and did not encourage media attention. As Namazi's detention stretches on, however, many people close to him now worry Iran may plan to hold him indefinitely.

A dual Iranian-American citizen, Namazi is a business consultant who had been based in Dubai for the past several years most recently as head of strategic planning at Crescent Petroleum Co., a privately owned oil and gas company in the United Arab Emirates. He has a bachelor’s degree in international relations from Tufts University and a master’s in urban and regional planning from Rutgers University. As a World Economic Forum Young Global Leader and a former scholar at the Wilson Center in Washington, D.C., he is among a group of educated and cosmopolitan Iranian-Americans who advocate for social and economic reform in their country.

Friends say that Namazi was headed back to Dubai in July when he was stopped at Tehran's airport and his passport confiscated. He was interrogated several times by the intelligence arm of Iran's Islamic Revolutionary Guard Corps (IRGC) before he was arrested and taken to Evin prison.

It is unclear whether Namazi has been formally charged, but Iranian media accounts depict him as a tool of "Zionists" and the West scheming to subvert Iran's regime. Iran's Supreme Leader has warned against economic "infiltration" by the U.S., and some analysts said Iranian hardliners want to send a signal to western-educated Iranian nationals that Iran remains closed for business to their ilk.

"Siamak Namazi has committed no crime. He was simply doing what he loved: building bridges between Iran and the Western world. There is no legal evidence to justify his detention, and he should be released immediately," says the letter sent to Kerry on Friday. It was signed by the Public Affairs Alliance of Iranian Americans; the Pars Equality Center; the National Iranian American Council; Iranian Alliances Across Borders; and the International Campaign for Human Rights in Iran.

Siamak Namazi was born in Iran, where his father was governor of the oil-rich Khuzestan province before the country's 1979 Islamic Revolution. His father left for the U.S. in 1983, becoming a United Nations official initially based in New York. Namazi returned to Iran in the mid-1990s, and in 1998 become a director at a consulting shop founded by Khajepour, Atieh Bahar Consulting, that advised companies looking to do business in Iran. At that time, Iran seemed poised for a possible rapprochement with the West.

Some Iran hawks in Washington criticized Namazi for his close ties to the regime during that period, but his connections were among Tehran's moderate factions, and in 2007, not long after the election of the hardline Iranian President Mahmoud Ahmadinejad, Namazi decamped to Dubai. (Khajepour was imprisoned during Iran's June 2009 Green Revolution and relocated to Vienna after his release.) Namazi's parents now live in Tehran.

Namazi's arrest accomplishes several goals for Iran's hardliners, including members of the IRGC opposed to President Hassan Rouhani's efforts to liberalize the country, said Karim Sadjadpour, a senior associate at the Carnegie Endowment for International Peace.

"It deters diaspora businessmen from visiting Iran, which is less competition for the IRGC. It signals to the U.S. that the nuclear deal was only about getting sanctions lifted and should not be misinterpreted as a desire to normalize relations. Lastly, it undermines the agenda of the Rouhani government, which had encouraged Siamak and people like him to return to Iran and bring with them foreign investment," Sadjadpour said.

"Many of his friends and former employees in the Iranian business community are doing non-stop media about how Iran is such a terrific place for foreign investment and don't even mention his name," he added. "It's shameful."

While aware that media attention can complicate negotiations, people close to Namazi fret that he has lacked the public advocacy enjoyed by several of the Americans released last month.

Rezaian's case was spotlighted by the Washington Post, and its billionaire owner, Amazon founder Jeff Bezos. U.S. Marine Amir Hekmati had prominent advocates in in the television host Montel Williams, who traveled to the nuclear talks in Vienna to publicize his case, as well as vigorous support from his congressman, Rep. Dan Kildee (D-Mich.). Fox News host Greta van Susteren closely covered the plight of imprisoned Christian pastor Saeed Abedini. All were released on January 16, along with Matthew Trevithick, whose arrest had not been publicized, and Nosratollah Khosravi-Roodsari, about whom little is known.

Also outspoken is the family of the other American citizen detained in Iran who has not been released, Robert Levinson. The former FBI agent disappeared in Iran in March 2007, reportedly while working as a CIA contractor. "We feel abandoned," Levinson's son told NBC News last month. Iran says it does not know where Levinson is.

Compounding the pain for Namazi's friends and family was a report from an Iranian news agency on the day of last month's prisoner swap inaccurately naming Namazi as among those freed. His name circulated widely on social media before it became clear that he would remain behind bars.